1. Field of Invention
The present invention relates to a silicon carbide semiconductor device, and a method for producing the same. Particularly, the invention relates to a silicon carbide semiconductor device characterized by an ohmic electrode formed on a lower surface of silicon carbide, and a method for producing the same.
2. Description of the Related Art
Silicon carbide (SiC) is superior to silicon (Si) in terms of properties including a wider band gap and a higher breakdown strength. Thus, use of silicon carbide (SiC) as a substrate material enables fabrication of a high-voltage and low-resistance power semiconductor device, beyond the limits of silicon (Si).
Another property of silicon carbide (SiC) is that it allows for formation of an insulating film by thermal oxidation, as does silicon (Si). For these reasons, use of silicon carbide (SiC) as a substrate material is considered to provide a high-voltage and low on-resistance MOSFET. Many studies and developments are underway regarding the use of silicon carbide (SiC) as a substrate material.
To this time, the developments of MOSFET have been conducted on the (0001) plane, because of the availability of the relatively inexpensive and highly crystalline epitaxial wafers. However, because the channel mobility at the MOS interface is low on this surface, it has been difficult to realize low on-resistance. Furthermore, there have been reports that an atmosphere of thermal oxidation has a large influence on the MOS interface channel mobility in the (000-1) and (11-20) planes, situated beneath and beside the (0001) plane, respectively, and that that mobility in these (000-1) and (11-20) planes can be increased more than in the (0001) plane, when oxidation is performed in a wet atmosphere, as disclosed in Non-Patent Documents 1 and 2 (see below). Thus, using these (000-1) and (11-20) planes is expected to realize a low on-resistance MOSFET.
In a producing method of MOSFET on the (000-1) plane, as described in Patent Document 1, an insulating film is formed by wet oxidation, and the interface state between the insulating film and silicon carbide is terminated with hydrogen. This is followed by vapor deposition of, for example, nickel, in a contact hole formed on an upper surface, and a subsequent heat treatment in hydrogen-containing gas. A result is a production of an ohmic electrode that has both high channel mobility and low contact resistance. In this procedure, it is a common practice to simultaneously perform the heat treatment for the surface electrode, and the ohmic electrode is formed on the lower surface.
The ohmic electrode formed on the lower surface requires not only low contact resistance but high adhesion strength for soldering. In one technique to achieve both low contact resistance and high adhesion strength, the electrode material is prepared from an alloy of nickel and metals of Group IVa, Va, and VIa, such as, for example, titanium, as described in Patent Document 2. In this way, a carbon layer that deposits on the silicide surface during the heat treatment of the alloy forms a carbide with the Group IVa, Va, and VIa metals such as titanium, so as to suppress the deterioration of the adhesion strength due to the carbon layer.    Patent Document 1: Japanese Patent Application 2006-060451.    Patent Document 2: JP-A-2000-208438.    Non-Patent Document 1: Fukuda et al. Applied Physics Letters, Vol. 84, pp. 2088-2090.    Non-Patent Document 2: Senzaki et al. Electron Device Letters, Vol. 23, pp. 13-15.